1. Field of Invention
This invention relates to a process for preparing membranes. More particularly, the invention relates to an improved process for the production of polybenzimidazole membranes which may be used, for example, in reverse osmosis.
2. Prior Art
Semipermeable membranes proposed in the past have been formed from a variety of materials, and are characterized by the ability to permit one component (e.g., ions or molecules) of a solution to pass through the membrane to the substantial exclusion of other components (e.g., other ions or molecules). Examples of substances heretofore recognized to possess this property include cellophane (i.e., regenerated cellulose), cellulose esters (e.g., cellulose acetate, cellulose butyrate, etc.), animal or protein membranes, polyelectrolyte complexes, ethyl cellulose, cross-linked polyacrylates, etc.
The semipermeable membranes of the prior art are of limited applicability in many separatory processes, such as reverse osmosis or ultrafiltration because of the inherent disadvantages relating to their chemical stability, strength, thermal stability, efficiency, length of life, and cost. Generally, the prior art membranes exhibit low thermal stability and, therefore, cannot be used successfully under conditions wherein the liquid undergoing treatment is at an elevated temperature. This may be a decided disadvantage in situations where the components to be separated only exist in solution at higher temperatures, or when it is economically advantageous to separate components of a solution at elevated temperatures rather than going to the expense of cooling it. Furthermore, some membranes exhibit a decided decrease in efficiency upon an increase in temperature or pressure thereby limiting their range of applicability. Solvent susceptibility may be another factor affecting the use of a particular porous membrane in a separation process. Additionally, semipermeable membranes may be inappropriate for a particular application due to low solute rejection values or low flux.
Other factors which render the semipermeable membranes of the prior art of limited usefulness in reverse osmosis separatory processes include their limited strength and chemical resistance and their extremely short operating lives at high pressures and temperatures.
The chemical resistance properties of the prior art separatory membranes have been their greatest shortcoming. Although the separation of solutions containing only sodium chloride and water presents few chemical resistance problems to the prior art membranes, such pure solutions are rarely found. Many naturally occurring saline solutions contain materials which exhibit a degrading effect on previously known reverse osmosis membranes. For example, cellulose acetate and amide-linked polymers, such as those disclosed in U.S. Pat. No. 3,567,632, are subject to either base or acid catalized hydrolysis even in weakly basic or acidic solutions. Other compounds which may exhibit a degrading effect on the prior art membranes include formic acid, acetone and bisulfite ions.
In an attempt to overcome certain of the above-noted disadvantages of prior art membranes, membranes comprised of polybenzimidazole polymers have been developed. See, for example, U.S. Pat. Nos. 3,699,038, 3,720,607, 3,737,042, 3,841,492, 3,851,025, 4,020,142, 4,448,687 and 4,512,894. In some instances, these membranes are fragile and contained voids in the membrane structure that reduced their strength.
U.S. Pat. Nos. 3,699,038, 3,737,042, 3,841,492, 3,851,025, 4,448,687, and 4,512,894 all disclose processes for the preparation of polybenzimidazole semipermeable membranes that include an annealing step. Usually the annealing step is added to the membrane preparation process to improve the mechanical properties of those membranes. However, the annealing step increases the cost and the time required to produce semipermeable membranes. In addition, although annealing enhances the salt rejection properties of membranes, it has the undesirable effect of decreasing the flux of the membrane.
U.S. Pat. No. 3,642,720 discloses the preparation of polybenzimidazole films without employing an annealing step. However, it does not disclose the preparation of membranes or the use of a non-solvent in the casting solution to enhance the mechanical properties of films.
Accordingly, it is an object of this invention to prepare polybenzimidazole membranes.
It is another object of this invention to prepare polybenzimidazole membranes without employing an annealing step.
It is also an object of this invention to prepare polybenzimidazole membranes which exhibit high flux and high salt rejection properties.
It is a further object of this invention to prepare polybenzimidazole membranes that are useful in reverse osmosis.
These and other objectives are obtained by employing the process of the instant invention.